1. Field of the Invention
The present invention relates to a magnetic encoder applied to a bearing having a function of detecting a rotational frequency or the direction of rotation, a wheel bearing having the magnetic encoder and a method of manufacturing a magnetic encoder.
2. Description of the Background Art
A magnetic encoder for detecting a rotational frequency or the like has such a structure that a magnetic member is circumferentially formed on an annular member and multipolarly magnetized in the circumferential direction. The magnetic encoder is rotated with a rotator so that a magnetic sensor closely opposed to the magnetic member detects the rotation thereby detecting the rotational frequency.
In general, the magnetic member of such a magnetic encoder is magnetized by one-shot magnetization or index magnetization.
FIG. 17 is a diagram for illustrating a method of performing one-shot magnetization on a magnetic member of a magnetic encoder. Referring to FIG. 17, a magnetic encoder 103 consisting of a metal ring 102 forming an annular member and a magnetic member 101 provided on the outer peripheral surface of the metal ring 102 is prepared. A magnetization yoke 111 supporting a plurality of exciting coils 112 is so arranged that the exciting coils 112 are opposed to the surface of the magnetic member 101 of the magnetic encoder 103. In this state, a current is fed to the exciting coils 112 in a prescribed direction, thereby magnetizing the magnetic member 101 and multipolarly magnetized in the circumferential direction.
FIG. 18 is a diagram for illustrating a method of performing index magnetization on a magnetic member of a magnetic encoder. Referring to FIG. 18, a magnetic encoder 203 consisting of a metal ring 202 forming an annular member and a magnetic member 201 provided on the outer peripheral surface of the metal ring 202 is prepared. A pair of tooth profiles of a magnetization yoke 211 having an exciting coil 212 wound thereon are closely arranged on the outer peripheral surface of the magnetic encoder 203. In this state, a current is fed to the exciting coil 212 for generating a magnetic flux passing through the magnetization yoke 211 in a prescribed direction, and the magnetic flux passing through the clearance between the tooth profiles magnetizes the magnetic member 201 for obtaining a pair of N and S poles. Thereafter a step of rotating the magnetic encoder 203 by a prescribed angle and magnetizing the same is repeated thereby multipolarly magnetizing the overall periphery of the magnetic member 201 in the circumferential direction. This method is hereinafter referred to as surface layer magnetization.
In the one-shot magnetization shown in FIG. 17, however, the magnetization pitch is disadvantageously largely irregularized depending on the manufacturing accuracy for each magnetic pole or the way of winding the coil when the pitch is reduced to not more than a pole width of about 1.5 mm, although high magnetization strength is attained.
In the surface layer magnetization shown in FIG. 18, the magnetization yoke 211 has a pair of tooth profiles and hence magnetization pitch accuracy is improved when attaining indexing accuracy of a spindle for rotating a workpiece. In this method, however, only the surface layer of the magnetic member 201 can be magnetized, disadvantageously leading to small magnetization strength.
An object of the present invention is to provide a magnetic encoder having high magnetization strength similar to that in one-shot magnetization and a small magnetization pitch error similar to that in surface layer magnetization.
Another object of the present invention is to provide a wheel bearing having a magnetic encoder having high magnetization strength and a small magnetization pitch error.
A magnetic encoder according to the present invention comprises an annular member and a magnetic member circumferentially provided on the annular member and multipolarly magnetized in the circumferential direction, and is characterized in that a magnetization pitch error of the magnetic member is not more than 3% and surface magnetization strength per unit magnetic pole is at least 30 mT/mm.
Then, surface magnetization strength per unit magnetic pole is a value obtained by dividing a maximum surface magnetization strength in one magnetic pole by the magnetic pole width.
Thus, it is possible to obtain a magnetic encoder having a small magnetization pitch error of not more than 3% similarly to that obtained in surface magnetization and high surface magnetization strength of at least 30 mT/mm similarly to that obtained in one-shot magnetization. Thus, the magnetic encoder can detect a rotational frequency or the like with higher accuracy.
In the aforementioned magnetic encoder, the annular member preferably consists of a magnetic substance, and the magnetic member preferably consists of an elastomer mixed with magnetic powder.
The annular member consists of a magnetic substance as described above, so that leakage of a magnetic flux can be suppressed in magnetization of the magnetic member and magnetization strength of the magnetic member is improved.
A wheel bearing according to the present invention rotatably supporting a wheel has a rotating member provided with the aforementioned magnetic encoder, and is characterized in that the rotating member is so arranged that the magnetic encoder is closely opposed to a magnetic sensor for detecting the speed of rotation of the wheel.
Thus, a wheel bearing provided with a magnetic encoder capable of rotating the speed of rotation of a wheel in high accuracy can be obtained.
The aforementioned wheel bearing preferably has a fixed member rotatably supporting the rotating member, and the magnetic encoder preferably forms a sealing apparatus sealing an annular space between the rotating member and the fixed member.
Thus, the magnetic encoder can also serve as the sealing apparatus, whereby increase of the number of components can be prevented.
In the aforementioned wheel bearing, the sealing apparatus preferably has a sealing member mounted on the fixed member to be capable of coming into sliding contact with the annular member of the magnetic encoder.
Thus, it is possible to prevent leakage of oil from the bearing or penetration of foreign matter of moisture from outside the bearing.
A method of manufacturing a magnetic encoder according to the present invention, circumferentially providing a magnetic member on an annular member and magnetizing the magnetic member, arranges a magnetization yoke passing a magnetic flux for magnetizing the magnetic member to hold the magnetic member and the annular member so that the magnetic flux passes through and magnetizes the magnetic member in the portion held by the magnetization yoke for successively magnetizing the magnetic member along the circumferential direction thereby multipolarly magnetizing the magnetic member in the circumferential direction.
The magnetization yoke thus holds the magnetic member and magnetizes the same so that the magnetic flux passes through the magnetic member, whereby magnetization strength can be increased similarly to that obtained by one-shot magnetization. Further, the magnetic member is magnetized every magnetic pole, whereby a magnetization pitch error can be reduced similarly to that obtained in surface layer magnetization so far as indexing accuracy of a spindle can be attained.
Preferably, the aforementioned method of manufacturing a magnetic encoder makes magnetization conditions variable in N pole magnetization and S pole magnetization of the peripheral surface of the magnetic member 1.
Thus, N and S poles can be controlled to reach substantially identical magnetization strength by varying the magnetization conditions.
Preferably, the aforementioned method of manufacturing a magnetic encoder varies the value of a current fed to a coil wound on the magnetization yoke with the N pole magnetization and the S pole magnetization.
Thus, the magnetization strength can be controlled with the value of the current.
Preferably, the aforementioned method of manufacturing a magnetic encoder varies the number of turns of a coil wound on the magnetization yoke with the N pole magnetization and the S pole magnetization.
Thus, the magnetization strength can be controlled with the number of turns of the coil.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.